Conventionally, upon printing image data sensed by a digital camera using a printer, a method of connecting a personal computer and digital camera using a serial interface such as RS-232C or the like to download image data to the personal computer, a method of directly loading image data to the personal computer using a memory card incorporated in the digital camera, or the like is used. Image data fetched by the personal computer is transferred to a printer, which is connected to that personal computer via a parallel interface such as a Centronics interface or a serial interface, after it undergoes a data process corresponding to the print format of the printer, thus executing a print process.
In recent years, a high-speed serial bus such as a USB (universal serial bus), IEEE1394 (FireWire), or the like is used as a serial interface. To a single high-speed serial bus, a plurality of devices can be cascade-connected in series. When a device, which does not perform data transfer in practice, of a plurality of devices which are cascade-connected to a single serial bus, undergoes power saving control (power management), a physical layer circuit connected to the serial bus must always be active. This is because the cascade-connected devices serve as repeaters, and data cannot be relayed if the physical layer circuits that repeat data are not active all the time.
However, if the power saving control operates only the physical layer circuit of the serial bus in an arbitrary device, but completely shuts down other circuits, the control cannot activate that device if that device receives a data transfer request. For this reason, a device that executes power saving control while waiting for data reception cannot be completely shut down, and a method using a sleep state in which a CPU is ready for startup anytime is adopted. In this way, a device which serves as a relay node cannot undergo power control which shuts down a power supply of that device. Such limitation on power control poses a serious problem in a device such as a portable personal computer, which must strictly execute power control since it does not receive any external power supply and uses a battery as a power supply.